Connections between optical transmission paths may be made by terminating optical fibers with plugs and by holding two such plugs which terminate optical fibers to be connected in predetermined positions with respect to each other. One such connection arrangement is referred to as a biconic connector which is disclosed in U.S. Pat. No. 4,512,630 which was issued on Apr. 23, 1985 in the name of P. K. Runge. In it, each optical fiber is terminated in a plug or ferrule having a truncated conical shape. Two such plugs are disposed in a biconical sleeve with small diameter end portions of the plugs being adjacent to a center plane. Another optical fiber connector is AT&T's ST.RTM. connector which comprises a cylindrically shaped ferrule or plug that terminates an optical fiber. The plug is disposed within a cap and is biased outwardly. Two such plugs may be inserted into a sleeve having a longitudinal slot therein with the end faces of the plugs being in contact with each other or spaced apart by an attenuator.
Typically an end portion of an optical fiber which is inserted into a passageway in a ferrule, for example, is held assembled to the ferrule by epoxy. The ferrule which terminates an optical fiber may be made of any of several materials. For example, connector ferrules have been made of ceramic, plastic or glass materials. After the epoxy has cured, the end face of the end portion of each optical fiber which is terminated by the ferrule needs to be polished to remove excess epoxy and provide a surface for light to enter or exit the fiber. After an optical fiber has been inserted into a passageway of the plug, an end portion of the fiber which extends beyond the end face of the plug is cleaved. This is a relatively rough, imprecise operation which leaves a portion of fiber extending beyond the end face of the plug.
In order to achieve low loss, low reflectance connections, the end faces of the two plugs in which the optical fibers terminate need to have surfaces which are substantially normal to the longitudinal axes of the plugs and which may have optical fibers protruding slightly therefrom and being smoothly polished. Otherwise, the surfaces may be skewed to each other and/or surface roughness may cause the end faces of the fiber cores not to be substantially in engagement with each other or in engagement with an attenuator that may be disposed between the end faces. The protruding end must be polished so that an end face of the fiber is coplanar with or protrudes slightly from the end face of the plug.
Polishing is performed in order to remove scratches. Otherwise, scratches which are too large can scatter light and cause reflections. They interfere with the mating of connectors and prevent the achieving of low loss. The presence of dirt, cracks, chips, scratches or pits in the core region of the optical fiber can cause increased insertion loss or lead to premature failure due to the propagation of cracks under variable environmental or mechanical stress. What is sought is a connector in which a minimum of light is reflected. Of course, every time there is an interface between two connectors, some reflections occur. The quality of the polishing operation is the key to achieving low reflections. In fact, some manufacturers grade their products by the quality of the polishing.
In the prior art, polishing of end faces of connector plugs and fiber has been accomplished manually. A connector plug to be polished is mounted in a fixture and the fixture is moved in oscillating circular patterns with the end face of the fiber and subsequently the plug in engagement with a polishing surface of a predetermined grit size. Such a fixture which may be used to polish an end face of a truncated conically shaped connector plug is disclosed in U.S. Pat. No. 4,539,779 which was issued on Sep. 10, 1985 in the name of F. R. Weaver. Such fixtures are still commonly used by craftspersons when making fiber terminations in the field.
The manual polishing of fiber and connector plug end faces is not without problems. It should be apparent that such a procedure is subject to operator variations in pressure applied to the fixture and hence that between grit of the polishing surface and the end faces. Also, the length of time, the motion and the path along which the plug traverses may vary from plug to plug thus producing inconsistent results in fiber end face protrusion and extent of polish. Variations in polishing media, as well as internal stresses in fibers and external contamination can contribute to variations in the polished surface.
Also available in the prior art is apparatus for gang-polishing a plurality of connector plugs. Each of a plurality of plugs to be polished is mounted in a nest of a clamping ring. Then the clamping ring is moved desirably to cause ends of fibers protruding from the plugs to engage a polishing surface. The problems with such an apparatus are twofold. One problem is that when the fibers are cleaved, the length of fiber that extends beyond an end face of the plug varies significantly from plug to plug. When a plurality, for example, eight or twelve, are gang-polished, the fibers extending from several of the plugs may be longer and hence experience greater pressure as forces are applied to the clamping ring for the plurality of plugs. This greatly increases the possibility of cracking those fibers which extend farther from associated plug end faces than others. Secondly, the plugs may vary in length and yet be within prescribed tolerance limits. As a result of the variations in plug length, some of the plugs may be under-polished whereas others may be over-polished.
Of course, other more sophisticated polishing techniques in which the entire face of the terminating ferrule is polished are available. However, the apparatus for carrying out such techniques is relatively expensive.
In the past, end faces of fiber terminating devices which have been prepared with the hereinbefore described apparatus have been inspected by human vision under microscopes. Such an inspection is not one hundred percent reliable inasmuch as it depends on the visual acuity of a human being as well as on the quality of the optical apparatus which is used for inspection. Further, such inspection is very subjective. A production worker can apply rules but is very difficult to quantify the number of scratches in an end surface of a fiber, for example. Very fine cracks can occur which are difficult for human inspectors to detect, even with the best optical aids such as high power microscopes. Cracks are a critical defect inasmuch as they can propagate under mechanical or environmental stress and lead to premature, catastrophic failure of the interconnection. The lack of consistent quantitative measures for detecting defects has been a major obstacle to the development of quality standards for connectors.
It is desirable to improve manufacturing processes to the point where defects in connectorized optical fibers become so rare that 100% inspection is not required. Experience to date shows that 100% inspection for the final polishing of optical fiber connectors is still required.
What is sought after and what does not appear to be available in the prior art are methods and apparatus which are capable of reliably and consistently detecting, measuring and classifying defects in end surfaces of optical fiber connector elements. What is needed and what is not provided in the prior art is an objective measurement method for classifying and quantifying defects so that changes and improvements in processes can be accurately measured. Such methods and apparatus should be capable of being used for optical fiber connector elements which are made of any of a variety of materials.